Hello, Tom.
I agree that lonely semicolon looks bad.
Applied your suggestion for empty loop body (/* skip */).
Patch in first letter had while(true), but I removed it cause
I think it is uglier:
- `while(true)` was necessary for grouping read with `if`,
- but now there is single statement in a loop body and it is
condition for loop exit, so it is clearly just a loop.
Optimization is valid cause compare_exchange always store old value
in `old` variable in a same atomic manner as atomic read.
Tom Lane wrote 2017-05-25 17:39:
Sokolov Yura <funny.fal...@postgrespro.ru> writes:
@@ -382,12 +358,8 @@ static inline uint64
pg_atomic_fetch_and_u64_impl(volatile pg_atomic_uint64 *ptr, uint64
and_)
{
uint64 old;
- while (true)
- {
- old = pg_atomic_read_u64_impl(ptr);
- if (pg_atomic_compare_exchange_u64_impl(ptr, &old, old & and_))
- break;
- }
+ old = pg_atomic_read_u64_impl(ptr);
+ while (!pg_atomic_compare_exchange_u64_impl(ptr, &old, old & and_));
return old;
}
#endif
FWIW, I do not think that writing the loops like that is good style.
It looks like a typo and will confuse readers. You could perhaps
write the same code with better formatting, eg
while (!pg_atomic_compare_exchange_u64_impl(ptr, &old, old & and_))
/* skip */ ;
but why not leave the formulation with while(true) and a break alone?
(I take no position on whether moving the read of "old" outside the
loop is a valid optimization.)
regards, tom lane
With regards,
--
Sokolov Yura aka funny_falcon
Postgres Professional: https://postgrespro.ru
The Russian Postgres Company
From d377e36da652ade715b868a6fc7164736f38d1df Mon Sep 17 00:00:00 2001
From: Sokolov Yura <funny.fal...@postgrespro.ru>
Date: Thu, 25 May 2017 14:54:45 +0300
Subject: [PATCH] Fix performance of Atomics generic implementation
pg_atomic_compare_exchange_*_impl stores old value into `old` variable,
so there is no need to reread value in loop body.
Also add gcc specific __sync_fetch_and_(or|and), cause looks like
compiler may optimize code around intrinsic better than around assembler,
although intrinsic is compiled to almost same CAS loop.
---
src/include/port/atomics/generic-gcc.h | 36 +++++++++++++++++
src/include/port/atomics/generic.h | 72 ++++++++++++----------------------
2 files changed, 60 insertions(+), 48 deletions(-)
diff --git a/src/include/port/atomics/generic-gcc.h b/src/include/port/atomics/generic-gcc.h
index 7efc0861e7..79ada94047 100644
--- a/src/include/port/atomics/generic-gcc.h
+++ b/src/include/port/atomics/generic-gcc.h
@@ -185,6 +185,24 @@ pg_atomic_fetch_add_u32_impl(volatile pg_atomic_uint32 *ptr, int32 add_)
}
#endif
+#if !defined(PG_HAVE_ATOMIC_FETCH_AND_U32) && defined(HAVE_GCC__SYNC_INT32_CAS)
+#define PG_HAVE_ATOMIC_FETCH_AND_U32
+static inline uint32
+pg_atomic_fetch_and_u32_impl(volatile pg_atomic_uint32 *ptr, uint32 and_)
+{
+ return __sync_fetch_and_and(&ptr->value, and_);
+}
+#endif
+
+#if !defined(PG_HAVE_ATOMIC_FETCH_OR_U32) && defined(HAVE_GCC__SYNC_INT32_CAS)
+#define PG_HAVE_ATOMIC_FETCH_OR_U32
+static inline uint32
+pg_atomic_fetch_or_u32_impl(volatile pg_atomic_uint32 *ptr, uint32 or_)
+{
+ return __sync_fetch_and_or(&ptr->value, or_);
+}
+#endif
+
#if !defined(PG_DISABLE_64_BIT_ATOMICS)
@@ -223,6 +241,24 @@ pg_atomic_fetch_add_u64_impl(volatile pg_atomic_uint64 *ptr, int64 add_)
}
#endif
+#if !defined(PG_HAVE_ATOMIC_FETCH_AND_U64) && defined(HAVE_GCC__SYNC_INT64_CAS)
+#define PG_HAVE_ATOMIC_FETCH_AND_U64
+static inline uint64
+pg_atomic_fetch_and_u64_impl(volatile pg_atomic_uint64 *ptr, uint64 and_)
+{
+ return __sync_fetch_and_and(&ptr->value, and_);
+}
+#endif
+
+#if !defined(PG_HAVE_ATOMIC_FETCH_OR_U64) && defined(HAVE_GCC__SYNC_INT64_CAS)
+#define PG_HAVE_ATOMIC_FETCH_OR_U64
+static inline uint64
+pg_atomic_fetch_or_u64_impl(volatile pg_atomic_uint64 *ptr, uint64 or_)
+{
+ return __sync_fetch_and_or(&ptr->value, or_);
+}
+#endif
+
#endif /* !defined(PG_DISABLE_64_BIT_ATOMICS) */
#endif /* defined(HAVE_ATOMICS) */
diff --git a/src/include/port/atomics/generic.h b/src/include/port/atomics/generic.h
index 424543604a..75ffaf6e87 100644
--- a/src/include/port/atomics/generic.h
+++ b/src/include/port/atomics/generic.h
@@ -170,12 +170,9 @@ static inline uint32
pg_atomic_exchange_u32_impl(volatile pg_atomic_uint32 *ptr, uint32 xchg_)
{
uint32 old;
- while (true)
- {
- old = pg_atomic_read_u32_impl(ptr);
- if (pg_atomic_compare_exchange_u32_impl(ptr, &old, xchg_))
- break;
- }
+ old = pg_atomic_read_u32_impl(ptr);
+ while (!pg_atomic_compare_exchange_u32_impl(ptr, &old, xchg_))
+ /* skip */;
return old;
}
#endif
@@ -186,12 +183,9 @@ static inline uint32
pg_atomic_fetch_add_u32_impl(volatile pg_atomic_uint32 *ptr, int32 add_)
{
uint32 old;
- while (true)
- {
- old = pg_atomic_read_u32_impl(ptr);
- if (pg_atomic_compare_exchange_u32_impl(ptr, &old, old + add_))
- break;
- }
+ old = pg_atomic_read_u32_impl(ptr);
+ while (!pg_atomic_compare_exchange_u32_impl(ptr, &old, old + add_))
+ /* skip */;
return old;
}
#endif
@@ -211,12 +205,9 @@ static inline uint32
pg_atomic_fetch_and_u32_impl(volatile pg_atomic_uint32 *ptr, uint32 and_)
{
uint32 old;
- while (true)
- {
- old = pg_atomic_read_u32_impl(ptr);
- if (pg_atomic_compare_exchange_u32_impl(ptr, &old, old & and_))
- break;
- }
+ old = pg_atomic_read_u32_impl(ptr);
+ while (!pg_atomic_compare_exchange_u32_impl(ptr, &old, old & and_))
+ /* skip */;
return old;
}
#endif
@@ -227,12 +218,9 @@ static inline uint32
pg_atomic_fetch_or_u32_impl(volatile pg_atomic_uint32 *ptr, uint32 or_)
{
uint32 old;
- while (true)
- {
- old = pg_atomic_read_u32_impl(ptr);
- if (pg_atomic_compare_exchange_u32_impl(ptr, &old, old | or_))
- break;
- }
+ old = pg_atomic_read_u32_impl(ptr);
+ while (!pg_atomic_compare_exchange_u32_impl(ptr, &old, old | or_))
+ /* skip */;
return old;
}
#endif
@@ -261,12 +249,9 @@ static inline uint64
pg_atomic_exchange_u64_impl(volatile pg_atomic_uint64 *ptr, uint64 xchg_)
{
uint64 old;
- while (true)
- {
- old = ptr->value;
- if (pg_atomic_compare_exchange_u64_impl(ptr, &old, xchg_))
- break;
- }
+ old = ptr->value;
+ while (!pg_atomic_compare_exchange_u64_impl(ptr, &old, xchg_))
+ /* skip */;
return old;
}
#endif
@@ -357,12 +342,9 @@ static inline uint64
pg_atomic_fetch_add_u64_impl(volatile pg_atomic_uint64 *ptr, int64 add_)
{
uint64 old;
- while (true)
- {
- old = pg_atomic_read_u64_impl(ptr);
- if (pg_atomic_compare_exchange_u64_impl(ptr, &old, old + add_))
- break;
- }
+ old = pg_atomic_read_u64_impl(ptr);
+ while (!pg_atomic_compare_exchange_u64_impl(ptr, &old, old + add_))
+ /* skip */;
return old;
}
#endif
@@ -382,12 +364,9 @@ static inline uint64
pg_atomic_fetch_and_u64_impl(volatile pg_atomic_uint64 *ptr, uint64 and_)
{
uint64 old;
- while (true)
- {
- old = pg_atomic_read_u64_impl(ptr);
- if (pg_atomic_compare_exchange_u64_impl(ptr, &old, old & and_))
- break;
- }
+ old = pg_atomic_read_u64_impl(ptr);
+ while (!pg_atomic_compare_exchange_u64_impl(ptr, &old, old & and_))
+ /* skip */;
return old;
}
#endif
@@ -398,12 +377,9 @@ static inline uint64
pg_atomic_fetch_or_u64_impl(volatile pg_atomic_uint64 *ptr, uint64 or_)
{
uint64 old;
- while (true)
- {
- old = pg_atomic_read_u64_impl(ptr);
- if (pg_atomic_compare_exchange_u64_impl(ptr, &old, old | or_))
- break;
- }
+ old = pg_atomic_read_u64_impl(ptr);
+ while (!pg_atomic_compare_exchange_u64_impl(ptr, &old, old | or_))
+ /* skip */;
return old;
}
#endif
--
2.11.0
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